In an optical fiber Local Area Network (LAN), a major figure of merit is the total number of stations that can be put on the network without an active repeater. In most cases, this is limited by the requirement that sufficient optical power be delivered from each transmitter to each receiver on the bus. In an LAN of the type considered, a transmitter 12 and receiver 13 are connected to an optical fiber bus 11 by way of an optical fiber tap 10 as indicated in FIG. 1 of the drawings. In order to couple the power from each transmitter to all of the receivers in the LAN, it is desirable to have each transmitter couple into the bus with unity efficiency and each receiver couple to the bus with a coupling efficiency of order 1/N, where N is the number of stations on the bus. Unfortunately, the principle of reciprocity forbids this, since it demands these two coupling coefficients be equal in passive couplers. This leads to very inefficient coupling, a large waste of optical power, and severe limitations on the number of stations that can be coupled to the bus.
It was recently proposed that this problem could be overcome in multimode optical fiber busses through the use of mode-selective coupling between a single-mode fiber and the multimode bus fiber. See the article entitled "Increased power injection in multimode optical fiber busses through mode-selective coupling," IEEE Journal Lightwave Technology, LT-3 537 (1985), by T. H. Wood. By coupling the single-mode optical fiber strongly to one or a few modes of the bus fiber, the light from the local transmitter can be efficiently coupled into the bus fiber. However, if the light traveling on the bus has its power uniformly distributed among the modes of the bus, this power will only be weakly tapped out. Calculations indicated that this approach can lead to an increase in injected power of about 7 dB, and an almost doubling of the number of possible stations on the bus. This coupling was experimentally demonstrated in a polished evanescent directional coupler between a 50 .mu.m core multimode fiber and a standard AT&T 5D single-mode fiber. See the article entitled "Effectively nonreciprocal evanescent-wave optical-fiber directional coupler", Electronic Letters, 21, 175 (1985), by M. S. Whalen and T. H. Wood. An input coupling efficiency of -0.2 dB and an output coupling efficiency of -18 dB were demonstrated.
However, this approach described hereinabove, has the serious drawback that it is only applicable to multimode fiber busses. Future LANs may use single-mode fiber, where much higher bandwidths are possible.